The objective of the project has been to assess the behavioral and clinical effects of low-level PCB exposure on infant rhesus monkeys. The experimental paradigm has been to expose breeding mothers to low levels of two commercial PCB preparations in their diets through gestation and nursing and then to examine the effects on the offspring. The reproductive difficulties experienced by the mothers are being studied on other grants. The earliest and highest-dosed group of infants (2.5 ppm Aroclor 1248 daily in the maternal diet) had previously shown hyperactivity and reversal-learning deficits which appeared correlated with the PCB residues measured in their mesenteric fat. In succeeding lower-dosed groups of infants, hyperactivity 4 times that of comparably aged control animals has been consistently observed, although the original group showed hypoactivity at 4 years of age. This latter phenomenon is currently under closer examination in the other groups. Discrimination-learning deficits were not nearly so pronounced in the lower-dosed groups and emphasis is now being given to the development of more sensitive behavorial assessment techniques, in discrimination learning, social observation and locomotor activity measurement. Theoretical development of basic concepts involving toxic thresholds is also under way.